Friction material such as that used in brake lining, clutch pads, and the like has severe performance requirements. The principal function of a friction element like a brake lining is to conert kinetic energy to heat and to absorb the heat or otherwise dissipate it while simultaneously through the agency of friction to reduce relative movement between the friction material and a part engaged by it. To achieve these objectives, it is necessary that the coefficient of friction between the friction material and the part so engaged be as reasonably high as possible, independent of variations in operating conditions, and accomplish the necessary energy conversion with a minimum wear of contacting parts. In particular, a friction material should not only have a relatively high coefficient of friction, but as well possess durability, heat stability, generate little or no noise while in rubbing contact with an engaging part, such as a rotor, and cause a minimum of wear on the engaged part.
In general, a friction material contains a matrix or binder, such as a thermosetting resin or vulcanized rubber, a fibrous reinforcement, and a friction modifier which aids in imparting a desired coefficient of friction to the material. Often the friction material may also contain fillers or extenders which modify its physical characteristics and reduce its cost.
Many frictional elements like brake linings have a tendency toward "fade", which is the inability to maintain a reasonably constant value of friction during repeated application of a brake system at a given speed, usually due to overheated brake linings. All linings exhibit fade to some extent. The inclusion of heat conducting particles, such as metal particles, in the friction material has been suggested to reduce the tendency to fade. However, this shortcoming still remains a serious problem.
Asbestos has been commonly used as a fibrous reinforcement for friction materials. However, asbestos is strongly suspected of being a carcinogen. Tiny fibers of asbestos are trapped by the mucous in the air passages of a human body, since the fibers are substantially insoluble and cannot be readily eliminated. Accordingly, an asbestos substitute has been sought which avoids the toxicity and potential medical liability attendent the use of asbestos in friction materials.
It has been suggested to substitute other fibrous materials for asbestos. For example, U.S. Pat. No. 2,861,964 to Gaugue, Jr., discloses the use of cellulosic fibers, such as wood fiber, as reinforcement for a composition brake block. U.S. Pat. No. 3,896,075 to Longley teaches the use of basalt fibers in complete substitution for asbestos fibers in friction materials. Basalt is a term applied to a group of volcanic rocks and defined as having a silica content of between 45% and 50%.
Glass has also been suggested as a component of friction material. U.S. Pat. No. 2,158,337 to Rasmussen discloses a brake lining material containing a glass fabric. Although there are thousands upon thousands of different glass compositions, no glass composition at all is disclosed in this patent for the purpose intended. Actually, glass fibers would not normally be considered effective in friction material, because they melt readily and fuse and become less viscous to smear or run-over companion parts, particularly at the brake interface, thereby seriously adversely effecting performance, especially fade.
In fact, the inherent tendency of a silicate glass under the influence of heat to fuse and smear, is used in U.S. Pat. No. 3,844,800 to Hooton to control wear of a friction material by controlling the formation of a surface glaze on the friction material which is created by the thermal energy generated during engagement with a corresponding friction element. Even in this instance, it is to be noted that the silicate glass powder is used in conjunction with a crystalline ceramic powder.
Normally, if glass is to be used in any form in friction material, one would choose a crystallized glass to avoid the problems of fusing, smearing, and run-out of amorphous glass when heat is produced by the friction material during use. This application of crystalline or ceramic glass is typified by U.S. Pat. No. 2,966,737 to Spokes et al, which relates to friction elements containing fibers which wear away without undergoing fusion in whole or in part and without undergoing any change tending to reduce friction. The fibers of this patent are said to remain dimensionally stable during use and, under high energy applications, do not undergo any significant chemical or mineralogical change. Heat may be applied during a sintering process encountered in production of the friction element or within the temperature limits of high energy braking. The patent recognizes the destruction of the fibrous character of non-crystalline glass by forming vitreous, separable products which migrate and reduce friction and detract from the essential fibrous nature of the fibers as originally incorporated in the friction element. To avoid this, the Spokes et al patent teaches forming fibers from sillimanite which is a member of the aluminim-silicate family crystallizing in the orthorhombic system.
British Pat. No. 1,445,975 is similar. This patent relates to friction material having non-vitreous fibers composed of a metal oxide salt such as a salt of alumina. When subjected to heat generated by the friction material, the alumina salt converts to a desired crystalline form, such as transitional and alpha aluminas, which consist of small crystallites, such that the alumina is ultimately used in the friction material in this crystalline fibrous form.